Observation: initial substitution occurs at the “alpha” position in naphthalene and other similar structures.
Rationalization: regioselectivity in electrophilic aromatic substitution depends on the stability of the cationic intermediate. If we compare the two competing pathways, we notice that we can write several resonance structures for each of the two cationic intermediates. However, the cation resulting from alpha addition (top) has one contributing structure that retains the unbroken benzene-like structure whereas the cation from beta addition (bottom) does not have any benzenoid contributing structures.
Because the benzenoid bonding pattern is associated with aromatic stabilization, we conclude that the preferred pathway of addition will be the one that retains the greatest number of intact benzene-like rings among its resonance structures.
Anthracene reacts at its inner ring (via cation A ) rather than either position on the outer rings (via

cations B or C ). We can understand this by looking at the resonance structures for these cation intermediates. You can see that cation A has one resonance contributor that preserves two benzenoid rings, but if you draw out the resonance contributors for the other possible cations (not shown) there is no such contributor possible. This tells us that cation A is the lowest energy ion and the product will be derived from this intermediate.
Electron-donating and withdrawing groups on polycyclic structures have an impact on the position of subsequent electrophilic aromatic substitution processes, as one would expect.
The first thing to observe is that electrophilic addition will occur in the most electron-rich ring of a polycyclic structure. Thus, an electron-withdrawing group will deactivate the ring it is attached to, while an electron-donating group wil activate the ring to which it is attached.
Now, what about the regioselectivity of addition to the other ring?
Recall that the basis for ortho/para and meta directing effects is whether the substituent is able to directly stabilize or destabilize the intermediate cation formed from addition at a particular site.
The same is true in polycyclic systems. Each possible site of addition leads to a cationic intermediate, and it is easy to assess how the substituent(s) interact with that particular intermediate. The cation that is most favoured will lead to the observed major product.
EWG
E
H
H
E
H
H
E
E H
EWG
E H
EWG
H
EWG
EWG
H
E
EWG
H
E
EWG
H
E
EWG
H
E
EWG
E H
EWG
E H
EWG
H
E H
EWG
H
EWG
H
H
E
H
EWG
H
H
E
EWG EWG
H
H
E
H
EWG
H
E
H EWG
EWG H EWG EWG
E H E H
H
E H E H
H
E H

In the chart above, all the resonance structures are shown for each of the four possible addition products to a naphthalene bearing an electron-withdrawing group at the alpha position. It can be seen that the first two cations avoid direct destabilization by the electron-withdrawing group, while the second pair are destabilized. We conclude therefore that the effect of the EWG is to favour addition at the following two positions:
EWG
E H
EWG EWG E EWG EWG
E
H
+
H
+
E
E
It is a lot of work to draw out all those resonance forms but it isn’t necessary to do so. Look at the products in the example above. Notice that the “E” group has added at the third and fifth carbons away from the EWG. This is exactly the same relationship as the two meta positions in a monocyclic compound! A similar argument can be made for electron-donating groups, which are ortho/para directors in monocyclic compounds. They are also ortho/para directing in polycyclic compounds.